(1) Field of the Invention
The present invention relates to mechanical seals and particularly to sleeve-like apparatus for establishing a seal about a tubular stub which extends from the wall of a vessel. More specifically, this invention is directed to a leakage repair technique and especially to a method for preventing leakage through or around a control rod nozzle support stub which extends into a nuclear reactor pressure vessel from the wall thereof. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well-suited for use in the field of boiling water type nuclear reactors (BWR). It has been discovered that, during operation of a BWR, leakage of liquid coolant from within the reactor pressure vessel may occur in the vicinity of the control rod nozzles. These nozzles are supported by means of stub tubes which are welded to the interior of the reactor pressure vessel. The nozzles are conventionally located below the reactor core and are sealed to the stub tubes by means of welds. The stub tubes, in turn, are welded to the pressure vessel. Should, by way of example only, a crack develop in a stub tube, the pressurized fluid within the reactor vessel may leak into the annular space between the stub tube and control rod nozzle and escape from the pressure vessel via this annular space. Since the liquid within the pressure vessel is directly exposed to the reactor core, it will be radio-active and any leakage thereof must be immediately terminated.
Copending application Ser. No. 434,092, filed Oct. 13, 1982, now U.S. Pat. No. 4,480,841, which is assigned to the assignee of the present invention and which is incorprated herein by reference, discusses in greater detail the above-mentioned problem of leakage resulting from reactor pressure vessel stub tube cracking. Application 434,092 also discloses novel apparatus which may be installed on a defective stub tube to prevent further leakage therethrough. The apparatus of the co-pending application performs its intended function efficiently and is useful in many applications.
In some reactor installations the sub tubes which support the control rod nozzles are very short, i.e., such stub tubes may for example be only one inch high. Further, while a stub tube crack appears to be the most prevelant possible cause of leakage about the control rod nozzles, it is possible that a crack could occur in the weld by which the stub tube is joined to the inner wall of the pressure vessel. The apparatus of the referenced co-pending application is not suitable for use with very short stub tubes and is not capable of correcting a leakage problem which occurs at or very close to the plane of the reactor pressure vessel wall.
It should also be noted that the establishment of a seal to the inner wall of a nuclear reactor pressure vessel is greatly complicated by the fact that such wall is provided with overlapping cladding strips which define an uneven surface. Again by way of example only, a BWR pressure vessel internal wall may be clad with stainless steel strips which are one inch wide and, in the regions of overlap between the strips, valleys approximately one eighth inch deep will be defined.